1. Field of the Invention
The invention relates generally to method and apparatus for packaging sensors. The invention relates more particularly to environmental sensors, such as pH sensors, where a microelectronic substrate such as an ion sensitive field effect transistor (ISFET) must be exposed to often caustic or acidic media.
2. Discussion of the Related Art
Various ion sensitive field effect transistors (ISFET's), or microelectronic ion sensors, are known in the art. Such ISFET's have advantages for use as pH sensors such as being solid state, small size and relatively inexpensive to produce.
However, certain problems arise in utilizing the ISFET as a practical solution for low cost sensing applications. Among these are the encapsulation of the device in a body or housing suitable for utilizing the ISFET as part of an ion sensitive probe for commercial purposes. Typically, the ISFET is potted in epoxy so that the sensor electronics are not subjected to the often severe environment of the liquid being tested. Epoxy can be hard to work with in cases where the sensor is not completely potted because the sensor must incorporate a window where the electroactive element is exposed to the liquid. Further problems arise in bonding the microelectronic ISFET structure to a suitable substrate for connection to additional probe electronics.
Also, epoxies are often not available with a wide range of choices against corrosion by various media and hence are not applicable over a wide range of environments to which probes must be impervious. More over, highly caustic solutions can etch the die itself, rendering it suitable for only a single use. Probe cost then becomes of paramount importance in single use applications.
A known packaging technique for a pH sensor 11 may be seen by referring to FIGS. 1A and 1B, which may also be seen in U.S. Pat. Nos. 5,068,205 and 4,851,104, respectively. In this technique a glass header 12 has been utilized wherein the silicon die (ISFET) 17 is adhered to a first side 14 of a borosilicate glass carrier 16 over a through-hole 15 therein. The carrier 16 has a through-hole 15 in it to maintain uncovered the contact areas of the ISFET 17. The carrier 16 also has leads, collectively 18, on the second side 20 thereof to provide electrical access to the ISFET area from the edges of the carrier. The ISFET substrate 17 is electrostatically bonded to the glass carrier 14. Lead wires, collectively 22, are then bonded between the ISFET and the glass carrier leads. The glass carrier leads 18 and back of the ISFET 17 are then covered with a second glass plate 24 for protection. This header assembly 12 is then connected to a flexible circuit 26 for leading out through the probe body 28. This header 12 and circuit 26 assembly are then enclosed within the probe body 28 along with a counterelectrode 27, as detailed in U.S. Pat. No. 4,851,104, and potted with epoxy to isolate the internal components from the typically corrosive liquids of the sensing environment. Yield is low and assembly costs are high due to the brittle nature of the glass substrate, the time consuming nature of epoxy potting and assembly of the many pieces.
Hence there is need for an ion sensitive microelectronic sensor package which is easily and inexpensively contained in an impervious housing while permitting media access to the die by hermetically sealing the probe electronics from the harsh media environment. More generally there is a need for packaging techniques for sensing apparatus which eliminates the drawbacks of epoxy potting.
Certain techniques for encapsulating piezoresistive pressure transducers with a conductive elastomeric seal, are detailed in U.S. Pat. No. 5,184,107 to Maurer. This patent details a low cost piezoresistive pressure transducer utilizing premolded elastomeric seals in which at least one seal is electrically conductive. A piezoresistive stress sensitive element in the form of a diaphragm of semiconductor material having a thickened rim is held at its rim between a pair of premolded elastomeric seals in a two piece housing. Electrical connections with external circuitry are made by conductive paths through one of the elastomeric seals which makes contact with electrical leads which pass through the housing wall.